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Routing First-Step: Internet addressing

Routing and Internet addressing are two important concepts involved in networking. Routing First-Step covers the basics of routing in clear, easy-to-understand language.

The following is the first installment of a multi-part series on the fundamentals of routing. Each tip is excerpted from Routing First-Step by William Parkhurst, published by Cisco Press. Check back frequently for the next installment, or go to the main series page for all the installments.

About the book

Routing First-Step explains the basics of Internet routing in language all of us can understand. This book takes you on a guided tour of routing, starting with systems you are familiar with: the postal system, the telephone system, and the interstate highway system. From there, you'll learn routing simply and easily. Whether you are looking to take your first step into a career in networking or are interested only in gaining knowledge of the technology, this book is for you!

Author William R. Parkhurst, Ph.D., CCIE, manages the CCIE Development group at Cisco Systems. The CCIE Development group is responsible for all new CCIE written qualification and laboratory exams. Prior to joining the CCIE team, Bill was a Consulting Systems Engineer supporting the Sprint Operation. He first became associated with Cisco Systems while a Professor of Electrical and Computer Engineering at Wichita State University. In conjunction with Cisco Systems, WSU established the first CCIE Preparation Laboratory.

Internet addressing

To begin our discussion on computer communication over a network, this section looks at the similarities between mail delivery between houses and data delivery between computers. The endpoints in mail delivery are houses, and the endpoints between electronic data delivery are computers.

Certainly there can be other endpoints in both systems. Letters can be delivered from a house to a business, from a business to a house, between two businesses, and so on. Electronic data delivery can be from a news service to your cell phone or personal data assistant (PDA), from your computer to your friend's pager, from environmental sensors in a building to the heating and cooling control systems for that building, and so on. But to keep the discussion simple, it will suffice to concentrate on mail delivery between houses, and electronic data delivery between computers. The first analogy is that an endpoint in a mail delivery system, a house, is equivalent to the endpoint in a computer communication system, a PC. (See Figure 3-1.)

Figure 3-1 - Equivalent Endpoints in the Mail and Data Communication Systems

In the mail delivery system, the function of the post office is to deliver mail to a particular house. In the computer communication system, the function of the Internet is to deliver data to a particular PC. Yet, in both systems, the endpoint is not the ultimate destination. For mail, the ultimate recipient is a person. For data, the ultimate recipient is an application such as an e-mail program, a web browser, an audio or video program, an instant messaging program, or any number of wonderful applications that exist today. (See Figure 3-2.)

Figure 3-2 - Final Destinations in the Postal and Electronic Data Delivery Systems

Although the ultimate recipient is a person or a software application, the responsibility of the systems stops when the mail, or data, is delivered to the proper house, or computer. However, as part of the address, you still need the ultimate recipient; either a person or an application, even though this information is not used for delivery to an endpoint. The endpoint uses the name or application to enable delivery to the recipient.

Because the two systems are analogous, it is instructive to revisit the format of an address in the mail delivery system and see if you can use a similar format for electronic data delivery:

Street Number, Street Name
City, State

Although there are five distinct pieces of information in the mail address (name, street number, street name, city, and state), you can consider an address to contain only four pieces of information. For endpoint delivery, you can ignore the name field. You are left with

Street Number
Street Name

The postal system routers (core, distribution, and access) use the state, city, and street names to deliver the mail from the source access post office to the destination access post office. The street number is not needed until the mail arrives at the access post office that is directly connected to the destination street. So, the address can be broken down into

State, City, Street Name
Street Number

The state, city, and street name information enables the mail to get close to the destination (a particular street). The street number is used to deliver the mail to the proper house. What is the analogy in the computer world to houses on a street? Recall from Chapter 1 that a group of computers can directly communicate with each other through a switch residing on a local-area network (LAN). So a LAN is the computer equivalent to a street. (See Figure 3-3.)

Figure 3-3 - LAN of Computers Is Similar to a Street of Houses

Before you learn more about Ethernet addresses, take the following quiz to make sure you understand the concepts described so far:

1. What number base is used to represent the Ethernet address?

Answer: Hexadecimal, because the symbols C and F are not used in the other number bases that we discussed. Computers compute using binary. The hexadecimal representation is for our benefit because it is easier to read and write.

2. How many bytes are in an Ethernet address?

Answer: Six. One hexadecimal digit contains 4 bits, or 1/2 bytes. Two hexadecimal digits contain 8 bits, or 1 byte. An Ethernet address contains 12 hexadecimal digits or 6 bytes.

3. How many bits are in an Ethernet Address?

Answer: 48 (8 bits per byte).

4. How many Ethernet addresses are possible?

Answer: 248 or 281 trillion, 474 billion, 976 million, 710 thousand, 6 hundred fifty-six (281,474,976,710,656).

An Ethernet address is not a property of your PC. An Ethernet address is a property of the Ethernet card, or built in Ethernet port in your PC. If you put a new Ethernet card in your PC, the Ethernet address of your PC changes.

By itself, an Ethernet address cannot deliver data between two endpoints on the Internet. The reason is that there is no structure to an Ethernet address. There are many manufacturers of Ethernet cards for computers, and each manufacturer is assigned a block of Ethernet address to use for their particular brand of card.

An analogy would be to have 281,474,976,710,656 postal addresses that are sold in a local postal address store. Each local postal address store is given a block of numbers from the total range of numbers that are possible. A postal address is just a number between 0 and 281,474,976,710,655. When you build a house, you would go to the local postal address store and your house would be assigned one of the numbers that hasn't yet been assigned. Everyone in your city would need to get a number assigned from the local postal address store. Because people will not be going to the store in any order, numbers will be assigned randomly throughout the city. The only way that these numbers can be used to deliver mail is if every post office at every level (core, distribution, and access) maintained a list of every number, and the route to reach that number. Therefore, every post office would need to maintain a list of 281,474,976,710,656 addresses and the route to get there. Obviously, this is not scalable. So in addition to an Ethernet address, you need another address that has a structure analogous to the structure of the postal address. What you need is an Internet addressing protocol.

All parts reproduced from the book Routing First-Step, ISBN 1587201224, Copyright 2005, Cisco Systems, Inc. Reproduced by permission of Pearson Education, Inc., 800 East 96th Street, Indianapolis, IN 46240. Written permission from Pearson Education, Inc. is required for all other uses. Visit for a detailed description and to learn how to purchase this title.

Chapter 1 also mentioned that computers have an address, and the most common technology used for computer communication is Ethernet. The sample Ethernet address that was presented in Chapter 1 was 00-03-47-92-9C-6F.
This was last published in September 2004

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